1. Field of the Invention
The present invention relates to a stick ignition coil apparatus for an internal combustion engine.
2. Description of Related Art
A stick ignition coil apparatus includes a center core, an outer core, a primary coil arrangement and a secondary coil arrangement. By switching on and off of supply of an electric current to the primary coil arrangement, a high voltage is generated in the secondary coil arrangement. The high voltage, which is generated at the secondary coil arrangement, is supplied to an ignition plug by way of a high voltage terminal, which is located at a (lower) end of the secondary coil arrangement.
As shown in FIG. 23A, a conventional stick ignition coil apparatus has a coil case 300, which includes a center core 302, a secondary coil arrangement 304, which is located radially outward of the center core 302, and a primary coil arrangement 308, which is located radially outward of the secondary coil arrangement 304. The secondary coil arrangement 304 includes a secondary spool 305 and a secondary winding 306, which is wound around the secondary spool 305. The primary coil arrangement 308 includes a primary spool 309 and a primary winding 310, which is wound around the primary spool 309.
A terminal plate (a second terminal element) 317, which is shaped into a tubular form, is installed at a bottom end of the secondary spool 305. A tubular-form high voltage terminal (a first terminal element) 315, an opening of which faces downward, is held in a spring case 312, which holds a spring 313. The spring 313 is located between the high voltage terminal 315 and the ignition plug (not shown). An upwardly extending portion of the high voltage terminal 315 contacts the terminal plate 317. Therefore, a bottom end of the secondary winding 306 is connected with the ignition plug, by way of the terminal plate 317, the high voltage terminal 315 and the spring 313. A space, which is formed by a bottom end of the coil case 300, the spring case 312, the high voltage terminal 315 and the like, is filled with dielectric resin 319 (see Unexamined Japanese Patent Publication No. 2000-133534).
However, in the above-described conventional art, a crack may be formed on the dielectric resin 319. Here, the terminal plate 317, which is a copper alloy, such as phosphor bronze, does not adhere well to the dielectric resin 319, which is made of a thermosetting epoxy resin. Furthermore, a coefficient of linear thermal expansion of the terminal plate 317 differs from that of the dielectric resin 319. Therefore, as shown in FIG. 23B, when a thermal stress is generated in a radial direction at the time of starting and stopping of an engine, and the crack tends to be formed at a phase boundary 321 between the terminal plate 317 and the dielectric resin 319.
An edge effect of the terminal plate 317 is added to the crack at the phase boundary. This means that the thermal stress is concentrated around an edge 317a and a resin portion therearound. Then, the dielectric resin 319 gets the crack 322, which starts from the edge 317a of the terminal plate 317. This may cause a dielectric breakdown.